This invention is directed to an absorbent structure including a fluid intake layer, a support layer, and a self-forming absorbent binder layer holding the fluid intake layer and the support layer together.
Adhesives, or binders, are a necessary element of many absorbent products. While adhesives beneficially hold products together, adhesives may also have a tendency to interfere with the absorbency of fluids in absorbent products. Adhesives are typically hydrophobic and therefore are not conducive to absorbency or liquid transfer functions. Furthermore, most adhesives are non-absorbent and thus serve no liquid retention function.
Hydrophilic adhesives are known, such as adhesives formulated from water-soluble polymers such as poly(vinyl alcohol), poly(vinyl methyl ether), poly(vinyl pyrrolidone), poly(ethylene oxide), or cellulose derivatives such as hydroxypropyl cellulose. Dextrans, starches and vegetable gums have been used to provide hydrophilic adhesives. These materials provide adhesion under dry conditions. However, upon exposure to aqueous fluids, these materials lose bonding capability because they are substantially soluble in aqueous fluids.
A known approach for making hydrophilic adhesives more functional upon exposure to aqueous fluid is to crosslink the water-soluble polymers. As a result of crosslinking, the material becomes swellable, and no longer soluble, in aqueous fluid. However, crosslinked polymers are difficult to apply to substrates or to establish intimate contact with surfaces because the crosslinked polymers are solid materials and have little or no ability to flow. Some of the crosslinked materials are fairly stiff, and inhibit the flexibility of the absorbent product.
What is therefore needed is a hydrophilic binder or coating that has latent crosslinking capability and which can be produced at attractive cost. Such binder or coating could be easily applied, like a water-soluble polymer, since the hydrophilic binder or coating would be capable of flow prior to crosslinking. Latent crosslinking capability would also provide a simple means of crosslinking the polymer after the polymer has established intimate contact with substrates or has formed a desired final shape or form. There is also a need or desire for such a binder which has a high level of flexibility.
Post-application crosslinking techniques are well known. Typical means of inducing the formation of crosslinks include high temperature xe2x80x9ccuringxe2x80x9d or exposure to radiation, such as ultraviolet or gamma radiation. Another known means of post-application crosslinking is moisture-induced crosslinking.
Recent development efforts have provided coating materials for a variety of uses. For example, U.S. Pat. No. 6,054,523, to Braun et al., describes materials that are formed from organopolysiloxanes containing groups that are capable of condensation, a condensation catalyst, an organopolysiloxane resin, a compound containing a basic nitrogen, and polyvinyl alcohol. The materials are reported to be suitable for use as hydrophobic coatings and for paints and sealing compositions.
Anderson et al., in U.S. Pat. No. 5,196,470, reported an alcohol-based, water-soluble binder composition. Because this composition is water-soluble and not cross-linked, it has no absorbency.
Others have reported the production of graft copolymers having silane functional groups that permitted the initiation of cross-linking by exposure to moisture. Prejean (U.S. Pat. No. 5,389,728) describes a melt-processable, moisture-curable graft copolymer that was the reaction product of ethylene, a 1-8 carbon alkyl acrylate or methacrylate, a glycidyl containing monomer such as glycidyl acrylate or methacrylate, onto which has been grafted N-tert-butylaminopropyl trimethoxysilane. The resulting copolymers were reported to be useful as adhesives and for wire and cable coatings.
Furrer et al., in U.S. Pat. No. 5,112,919, reported a moisture-crosslinkable polymer that was produced by blending a thermoplastic base polymer, such as polyethylene, or a copolymer of ethylene, with 1-butene, 1-hexene, 1-octene, or the like; a solid carrier polymer, such as ethylene vinylacetate copolymer (EVA), containing a silane, such as vinyltrimethoxysilane; and a free-radical generator, such as an organic peroxide; and heating the mixture. The copolymers could then be cross-linked by reaction in the presence of water and a catalyst, such as dibutyltin dilaurate, or stannous octoate.
U.S. Pat. No. 4,593,071 to Keough reported moisture cross-linkable ethylene copolymers having pendant silane acryloxy groups. The resultant cross-linked polymers were reported to be especially resistant to moisture and to be useful for extruded coatings around wires and cables. The same group has reported similar moisture curable polymers involving silanes in U.S. Pat. Nos. 5,047,476, 4,767,820, 4,753,993, 4,579,913, 4,575,535, 4,551,504, 4,526,930, 4,493,924, 4,489,029, 4,446,279, 4,440,907, 4,434,272, 4,408,011, 4,369,289, 4,353,997, 4,343,917, 4,328,323, and 4,291,136.
U.S. Pat. No. 5,204,404 to Werner reported crosslinkable hydrophobic acrylate ester copolymers including 0.1 to 10% acrylic acid. The resultant cross-linked polymers were reported to be useful for painting and refinishing the exterior of automobiles.
These examples of moisture-induced crosslinking are applied to substantially hydrophobic polymers. Since the cured products of these formulations are reported to be useful for coverings for wire and cable, and for non-conductive coatings for electrical conductors, and for painting and refinishing the exterior of automobiles, it would be expected that they are durable coatings for which properties such as water absorbency would be a disadvantage.
Conventional personal care absorbent articles including diapers, training pants, sanitary napkins, adult incontinence garments and the like, have a relatively complex structure and manufacturing procedure. Personal care absorbent articles typically include a liquid-permeable bodyside liner, a surge (compensation) layer which receives and distributes liquid received through the liner, a single-layer or multiple-layer absorbent core which receives and stores liquid that passes through the compensation layer, and a liquid-impermeable outer cover that prevents liquid in the absorbent article and provides a substantially dry outer surface. Each of these layers is separately manufactured. The layers are then combined using adhesive bonding, thermal bonding, ultrasonic bonding and other techniques which must be tailored to sufficiently bond the layers together without compromising their respective functions. There is a need or desire for absorbent articles having simpler structures that can be manufactured using simpler, less expensive techniques.
This invention is directed to a three-layer absorbent structure and to absorbent articles containing it. The three-layer absorbent structure includes the following layers, in sequence, with no additional adhesive or other layers in between them:
a) a liquid-permeable fluid intake layer,
b) a flexible absorbent binder layer, and
c) a support layer.
The flexible absorbent binder layer serves as a fluid storage (absorbent) layer and also bonds the fluid intake layer to the support layer. The absorbent article may contain additional layers, so long as the above three layers of the absorbent structure occur in sequence. In some absorbent articles, the three-layer absorbent structure may not be accompanied by additional layers.
The three-layer absorbent structure is formed by applying an absorbent binder composition to one or both facing surfaces of the fluid intake layer and the support layer, bringing the fluid intake layer and support layer together, and crosslinking the absorbent binder composition to form the flexible absorbent binder layer. Because the flexible absorbent binder layer is formed (crosslinked) while the absorbent binder composition is in contact with the other layers, the flexible absorbent binder layer serves both as an absorbent layer and an adhesive (binder) layer in the three-layer absorbent structure, eliminating the need for additional adhesive or other bonding steps.
The absorbent binder composition includes about 15 to about 99.9% by mass of monoethylenically unsaturated polymer units. Suitable monoethylenically unsaturated polymers include without limitation carboxylic acid, sulphonic acid, phosphonic acid, and salts of the foregoing. The absorbent binder composition also includes about 0.1 to about 20% by mass of acrylate or methacrylate ester units that include an alkoxysilane functionality. Upon exposure to water, the alkoxysilane functionality forms a silanol group which condenses to form a crosslinked polymer.
The absorbent binder composition may also include zero to about 75% by mass of polyolefin glycol and/or polyolefin oxide units. The polyolefin glycol and/or oxide may include an alpha-olefin having about 2 to about 4 carbon atoms, and may include about 30 to about 15,000 olefin glycol and/or oxide units per molecule. The polyolefin glycol and/or oxide may be graft polymerized with the acrylate or methacrylate ester to form a graft copolymer. The polyolefin glycol and/or oxide may be a homopolymer or copolymer. The polyolefin glycol and/or oxide may be a block copolymer including olefin glycol or oxide units having different numbers of carbon atoms, for instance, block copolymers of ethylene oxide and propylene oxide. The polyolefin glycol and/or oxide provides the absorbent binder composition with enhanced flexibility. Thus, the flexible absorbent binder layer has enhanced adhesion in a wet condition, absorbency, and flexibility.
The absorbent binder composition may be used to form a flexible absorbent binder layer on and between such layers as nonwoven webs, woven webs, foams, knitted fabrics, cellulose tissue, plastic film, stranded composites, staple fibers, yarns, elastomer net composites, or any other suitable substrates. Examples of plastic film substrates include those made of polypropylene, low density polyethylene, high density polyethylene, linear low density polyethylene, and ultra low density polyethylene. Examples of absorbent articles in which the three-layer absorbent structure may be used include diapers, diaper pants, training pants, feminine hygiene articles, swim wear, adult incontinence garments, swimwear garments, medical absorbent articles, and the like.
The absorbent binder composition can be prepared using a template polymerization process by which the monoethylenically unsaturated polymer and acrylate or methacrylate ester are polymerized in the presence of a pre-formed template polymer, which is the polyolefin glycol and/or polyolefin oxide. The polymerization can be carried out by reacting two different monoethylenically unsaturated monomers, one of which contains an alkoxysilane functionality. The polymerization may be induced by heat, radiation, redox chemical reactions, and other techniques. Suitable radiation initiators include without limitation ultraviolet, microwave, and electron beam radiation. The initiator generates free radicals to cause copolymerization of the monomers. In one embodiment, the polymerization reaction is carried out in an organic solvent such as ethanol. The polymerization may also occur in an aqueous solution, or in a combined aqueous and organic solvent.
The polyolefin glycol and/or oxide may or may not be graft polymerized onto the acrylate or methacrylate units during the polymerization process. The resulting absorbent binder composition may contain the polyolefin glycol and/or oxide as a separate component, or as part of the copolymer, or a combination of both.
The resulting absorbent binder composition has latent moisture-induced crosslinking capability due to the alkoxysilane functionality. The composition may be applied, in a flowable state, to the fluid intake layer and/or support layer. After the layers are brought together, moisture-induced crosslinking may be accomplished through hydrolysis of the alkoxysilane and subsequent condensation upon removal of the solvent from the substrate, either by evaporation of the solvent from the substrate or using any other effective technique. Alternatively, the hydrolysis of the alkoxysilane and subsequent condensation may occur after solvent removal by exposure of the coating to moisture in ambient air.
With the foregoing in mind, it is a feature and advantage of the invention to provide a three-layer, absorbent structure having a self-forming absorbent binder layer, and absorbent articles including the three-layer absorbent structure.
Within the context of this specification, each term or phrase below will include the following meaning or meanings.
xe2x80x9cBinderxe2x80x9d includes materials which are capable of attaching themselves to a substrate or are capable of attaching other substances to a substrate.
xe2x80x9cFeminine hygiene productsxe2x80x9d include sanitary pads and napkins, as well as tampons and interlabial feminine hygiene products.
xe2x80x9cFluidxe2x80x9d refers to a substance in the form of a liquid or gas at room temperature and atmospheric pressure.
xe2x80x9cHigh density polyethylene (HDPE)xe2x80x9d refers to a polyethylene having a density of about 0.95 g/cm3 or greater.
xe2x80x9cKnife over roll coatingxe2x80x9d refers to a process in which a knife is positioned, with a specified gap, above a substrate that is moving beneath the knife on a moving roll. In this manner, the knife spreads a specified thickness of coating material onto the substrate.
xe2x80x9cLayerxe2x80x9d when used in the singular can have the dual meaning of a single element or a plurality of elements.
xe2x80x9cLinear low density polyethylene (LLDPE)xe2x80x9d refers to polymers of ethylene and higher alpha-olefin comonomers such as C3-C12 comonomers, and combinations thereof, having a density of about 0.900 to about 0.935 g/cm3.
xe2x80x9cLow density polyethylene (LDPE)xe2x80x9d refers to a polyethylene having a density between about 0.91 and about 0.925 g/cm3.
xe2x80x9cModifying agentxe2x80x9d refers to a substance that may be added to a composition to modify the physical properties of the composition, such as the color or texture of the composition.
xe2x80x9cNonwovenxe2x80x9d or xe2x80x9cnonwoven webxe2x80x9d refers to materials and webs or material having a structure of fibers or filaments which are interlaid, but not in an identifiable manner as in a knitted fabric. The terms xe2x80x9cfiberxe2x80x9d and xe2x80x9cfilamentxe2x80x9d are used interchangeably. Nonwoven fabrics or webs have been formed from many processes such as, for example, meltblowing processes, spunbonding processes, air laying processes, and bonded carded web processes. The basis weight of nonwoven fabrics is usually expressed in ounces of material per square yard (osy) or grams per square meter (gsm) and the fiber diameters are usually expressed in microns. (Note that to convert from osy to gsm, multiply osy by 33.91.)
xe2x80x9cPersonal care absorbent productxe2x80x9d includes diapers, diaper pants, training pants, swim wear, absorbent underpants, adult incontinence products, feminine hygiene products, and the like.
xe2x80x9cRoll printingxe2x80x9d or xe2x80x9croll coatingxe2x80x9d refers to a process in which the application of a deposited material, generally as a paste, onto a substrate is carried out by transferring the deposited material from a roll onto the substrate in a more or less uniform layer using one or more rolls, which may be engraved, or a pool cylinder. A doctor blade is used to scrape any excess deposited material from the rolls or substrate. The doctor blade may be flat or have a patterned edge such as slots or ridges.
xe2x80x9cRotary screen printingxe2x80x9d or xe2x80x9crotary screen coatingxe2x80x9d refers to a process that is a combination of roll printing or coating and screen printing or coating.
xe2x80x9cScreen printingxe2x80x9d or xe2x80x9cscreen coatingxe2x80x9d refers to a method of applying a deposited material by forcing the material to be deposited through a screen that may have uniform openings or patterned openings.
xe2x80x9cStranded compositesxe2x80x9d refer to sheets of material to which strands of an elastomeric material are adhered to create an elastomeric composite.
xe2x80x9cSuperabsorbentxe2x80x9d refers to a water-swellable, water-insoluble organic or inorganic material capable, under the most favorable conditions, of absorbing at least about 10 times its weight and, more desirably, at least about 25 times its weight in an aqueous solution containing 0.9 weight percent sodium chloride. The superabsorbent materials can be natural, synthetic, and modified natural polymers and materials. In addition, the superabsorbent materials can be inorganic materials, such as silica gels, or organic compounds such as cross-linked polymers. A material is xe2x80x9cabsorbentxe2x80x9d if it absorbs at least five times its weight of the aqueous solution under these conditions.
xe2x80x9cUnitxe2x80x9d or xe2x80x9cpolymer unitxe2x80x9d refers to a monomer or polymer portion of a copolymer molecule or blend component that includes a different molecular structure, compared to another portion of the copolymer or blend.